Although there are many diseases which affect breathing and some of these are potentially fatal, a means to measure and monitor ventilation in spontaneously breathing patients for even short periods of time without rigid constraints on posture, is not known in the art. Long term ventilatory monitoring cannot be done by any known method unless a patient is intubated. This is a major problem. Patients are frequently admitted to intensive care units because the physician feels that intubation and mechanical ventilation may be required. A system that could measure and monitor ventilation would detect if the patient's ability to breathe is improving or deteriorating in time and would provide the physician with crucial information allowing him/her to make a better-informed decision. Using an alarm system, a respiratory arrest would result in almost instantaneous resuscitation because the potentially fatal event would be detected as soon as it occurs.
Babies, particularly premature ones, are at risk of the sudden infant death syndrome (SIDS) which is usually attributed to a prolonged respiratory arrest. It is known that interbreath interval is more variable in premature compared to normal term babies. Thus noninvasive measurement of ventilation could identify babies at risk for SIDS and high risk babies could be monitored by the system so that if a respiratory arrest occurred it could be identified and virtually immediately corrected.
Sleep disordered breathing is extremely common, but unfortunately no accurate means exist to measure and monitor breathing during sleep. Long term monitoring of ventilation could prove to be of great importance in diseases such as asthma and emphysema just as Holter monitoring is useful in detecting and diagnosing cardiac arrhythmias. Thus, clinical medicine has great need for a method to measure and monitor ventilation and its parameters so that respiratory arrest can be diagnosed and immediately treated; so that in patients at risk, deterioration of the ability to breathe can be detected early; so that infants at risk for SIDS can be identified and monitored; for the diagnosis of sleep-disordered breathing and for recovery of ventilatory abnormalities that had not previously been known to exist.
Precise monitoring of ventilation is presently achieved by having the subject breathe through a mouthpiece or face mask attached to a pneumotachygraph or spirometer. While these devices permit the accurate measurement of lung volumes, they also alter the pattern of breathing and the minute ventilation (V.sub.E). In addition, they are useful only for occasional measurement of respiratory parameters. Long term monitoring of ventilation requires a device which would permit freedom of movement and eliminate the need for a mouthpiece and nose clip.
More versatile means of measuring ventilation are presently available. These methods (Magnetometry and Respitrace.RTM.) record the movements of the ribcage and abdomen during respiration and, by the use of suitable calibrations, convert the summed thoracic and abdominal motions into a volume signal. While these devices eliminate the need for a mouthpiece and, as a result, permit measurement of volumes and timing parameters representative of normal breathing, their calibration changes with posture, particularly changes in xiphi-pubic distance.
Monitoring and measuring respiration "non-invasively" by converting a tracheal sound signal to an airflow signal has been studied by others in the past with limited success. Phonospirometry, the estimation of ventilation from measurements of tracheal breath sounds, provides a simple alternative to Respitrace.RTM. and magnetometry, and may prove to be more versatile. Since the invention of the stethoscope by Rene Laennec in 1819, auscultation has provided the clinician with a quick, if crude, assessment of pulmonary ventilation. Objective measurements of breath sounds were first made more than twenty-five years ago, but it is only with advances in computer technology and the wide application of digital signal processing or numerical analysis that recording and analysis of respiratory sounds has accelerated. Lung sounds can be either normal or adventitious. Normal lung sounds appear to be primarily generated by the complex turbulence within the large and medium sized airways. The sound characteristics are influenced by airflow velocity and the local properties of the airways. As a result, for a given subject and microphone position, sound intensity is proportional to the flow rate (Leblanc, P.; Macklem, P. T.; Ross, W. R. D. Breath sounds and distribution of pulmonary ventilation. American Review of Respiratory Disease 102:10-16;1970, (Shykoff, B. E.; Ploysongsang, Y. ; Chang, H. K. Airflow and normal lung sounds. American Review of Respiratory Disease 437:872-876;1988), (Graviely, N.; Cugell, D. W. Airflow effects on amplitude and spectral content of normal breath sounds. J. Appl. Physiol. 80:5-13, 1996)
Many investigators presently measure airfiow by acoustical techniques (Ajmani, A.; Mazumdar, J. ; Jarvis, D.--Spectral analysis of an acoustic respiratory signal with a view to developing an apnea monitor Australasian Physical & Engineering Sciences in Medicine 19:46-52;1996) (Makarenkov, A. P.; Rudnitskij, A. G.--Diagnosis of lung pathologies by two-channel processing of breath sounds Akusticheskii Zurnal 41:272-277;1994) (Pasterkamp H.; Kraman, S. S.; Wodicka, G. R. Respiratory sounds. Advances beyond the stethoscope. American Journal of Respiratory and Critical Care Medicine 156:974-987;1997) (Soufflet, G.; Charbonneau, G.; Polit, M.; Attal, P.; Denjean, A.; Escourrou, P.; Gaultier, C.--Interaction between tracheal sound and flow rate: a comparison of some different flow evaluations from lung sounds IEEE Transactions on Biomedical Engineering, vol. 37, No. 4, April 1990), however, their interest lies mainly in the derivation of parameters related to the frequency spectrum of the sound signal rather than in the use of the sound signal as a surrogate for flow. The papers by Shykoff et al. and Graviely and Cugell measure the relationship between sound and flow, only at flow rates greater than 0.5 lps. At flow rates less than 0.5 lps, almost no sound is detectable, and the background noise makes the signal-to-noise ratio very high. In Soufflet et al., the results reported would indicate that flows less than 0.5 lps were measured, however,. there is no indication how the problem of low flow rate can be solved.
In the known prior art, there is difficulty in recording sound intensity in the trachea and converting the sound signal into the appropriate flow signal. Typically, when the flow rate is low (e.g. less than 0.5 liters per second) conversion of the sound signal into a flow signal is not reliable enough for scientific or medical purposes. In the known prior art, there is no method for obtaining a reliable conversion of a tracheal sound signal into a flow signal in normal breathing which includes portions of low flow rates.